regulator: core: Allow dummy regulators for supplies

Rather than just not resolving the supply when there is explicitly no
supply mapping fall through and allow a dummy supply to be substituted.
This fixes issues with constant retries reported by Dong Aisheng.

Signed-off-by: Mark Brown <broonie@kernel.org>
Tested-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>